Intracranial pressure (ICP) is closely related to cerebral perfusion (blood flow in the brain). Elevated ICP reduces cerebral perfusion pressure, and if uncontrolled, results in vomiting, headaches, blurred vision, or loss of consciousness, escalating to permanent brain damage. Increased ICP is a medical/surgical emergency, and in severe cases, disability and death may occur.
Although several non-invasive techniques of measuring ICP have been proposed during the last decade, typically ICP monitoring is achieved by using sensors implanted within the cranium, or external sensors connected to the measurement site in the cranium with a fluid-filled catheter. These approaches are invasive, generating risk of intracranial infection and pain for the patient, and require neurosurgical expertise for their implantation. Moreover, long term monitoring of ICP often requires significant restriction on the movement of a patient. The most common invasive procedure is the lumbar puncture where a catheter with a pressure-sensing device is placed in the lumbar subarachnoid space.
Non-invasive assessment of ICP has been pursued with several approaches that are based on different physical principles. These approaches share the common idea of measuring an alternative physical variable that relates to the change of ICP. The accepted and commercially available technique for non-invasive estimation and monitoring of ICP consists of taking a computed tomography (CT) or other image of the head, interpreting the image and observing changes in various features. This method requires a high level of skill to read and assess the images and requires that the patient be brought to the imaging equipment. In many cases, a scan is delayed or put off simply because the patient is not stable enough to be moved. Even after the patient is stable, the various tubes and equipment connections to the patient have to be accounted for during the trip to the CT area, and additional personnel are often required, with a respective increase in cost. In addition, the scans themselves are single measurements, of which at least two are required to assess subtle-changes and variations. A series of scans could approximate continuous monitoring, but this is not practical.
Other non-invasive ICP monitoring techniques have been developed. A non-invasive ICP monitoring system is taught in U.S. Pat. No. 4,841,986. This system is based on fine volume measurements of the external auditory canal during elicitation of the human stapedial reflex. U.S. Pat. No. 5,919,144 discloses a non-invasive system based on real-time analysis of acoustic interaction with the brain and changes in tissue acoustic properties as ICP changes. Electromagnetic techniques are disclosed in U.S. Pat. No. 4,690,149. Ultrasonic or vibratory techniques are disclosed in U.S. Pat. No. 3,872,858. Jugular vein occlusion techniques are disclosed in U.S. Pat. No. 4,204,547. Another technique stated to be non-invasive utilizes a nuclear powered pressure sensor designed to be implanted totally under the scalp of the patient and is disclosed in U.S. Pat. No. 4,141,348. A method for determining ICP based on acoustic data from a target site is disclosed in U.S. Pat. No. 7,547,283.
Each of the currently used or medically accepted methods for ICP assessment is deficient in some way. Because of the deficits in current ICP measurement methodologies, there is a need for an easily administered non-invasive (or minimally invasive) method for continuous long-term monitoring of ICP.